1. Field of the Invention
This invention relates to magnetic flow sensing probes used in electromagnetic, or Faraday, flow meters.
2. Background Information
The rate of flow of a conductive fluid can be determined by measuring an electrical potential difference developed in the fluid as the fluid moves through a magnetic field. The potential difference is sensed by at least one pair of electrodes contacting the liquid and spaced apart from each other along a line that is nominally orthogonal to both the direction in which the flow is being measured and a magnetic field produced by a magnet. Some instruments of this sort are configured as in-line flow meters in which the electrodes and magnets are either built into or are very close to the wall of a pipe through which the fluid flows. Other instruments of this type are configured as insertion probes in which the electrodes are or adjacent a free end of the probe. When the probe is inserted into a flowing fluid the measurement is made at some selected distance from whatever pipe wall or other support member is used to anchor the probe in place.
A concern with flow probes in general is that a non-uniform fluid flow distribution, as may be caused by the flow impedance of the probe head, can cause the probe to vibrate. This vibration may lead to non-linear response or to mechanical failure.
A particular concern related to magnetic flow probes has been that fluid flow distribution in the neighborhood of the electrodes may not remain uniform because of the flow obstruction caused by the probe itself. At low flow rates the fluid tends to change course in the vicinity of the probe and may be deflected away from the electrodes so as to reduce the measured signal. This can result in a nonlinear relation between the flow-generated electrode signal and the flow rate even in the absence of perceptible vibration. Other nonlinearities may be caused at intermediate and higher velocities by the fluid impinging on portions of the probe distal from the electrodes. Additional nonlinearities due to the proximity of the pipe wall to the head may also occur.
Another concern is that in some magnetic flow probes the electrodes may be held in an electrically insulating head at the end of a stem. In these cases the electrically conductive area available for grounding the probe to the fluid may be limited to the stem portion of the probe. Unless such a probe is used in a relatively small diameter metallic pipe, this lack of grounding can lead to unstable and noisy operation.